1. Field of the Invention
The present invention relates to a light-emitting element driving circuit and a cellular phone, and more particularly to a light-emitting element driving circuit capable of changing the luminance of a light-emitting element, and a cellular phone incorporating the light-emitting element driving circuit.
2. Description of the Related Art
A trend of recent cellular phones is enabling users to view TV broadcasting programs and other videos on a main liquid crystal display screen. To this end, cellular phones are required to incorporate a light-emitting element driving circuit that can change the luminance of a backlight equipped in the liquid crystal display device. Meanwhile, excessive current consumption by the backlight of the main liquid crystal display device is a problem to be solved. To this end, there is a conventional method for solving the problem by changing the luminance of the backlight of the liquid crystal display device according to the content of a video signal. More specifically, the method includes enhancing the brightness by increasing the luminance of the backlight when the video signal is a bright image and enhancing the darkness by decreasing the luminance of the backlight when the video signal is a dark image. In this manner, the light-emitting element driving circuit is required to reduce wasteful current consumption and realize a long-term use of the battery.
For example, a light-emitting element driving circuit discussed in Japanese Laid-Open Patent Application No. 2005-11895 is a light emitting diode (LED) driving circuit including a battery that supplies drive current to an LED. A constant current circuit, which is disposed on an anode side or a cathode side of the LED, controls a current value of the current flowing through the LED to have a predetermined target value. A resistor is connected to the cathode side of the LED and a downstream side of the constant current circuit. When a sum of a voltage drop across the LED in a forward direction, a drive voltage of the constant current circuit attaining the predetermined target value, and a terminal voltage of the resistor applied when the predetermined target value is attained, is a predetermined voltage, the voltage of the battery varies according to a residual capacity within a range including the predetermined voltage value. A boosting circuit, which is connected between the battery and the LED, outputs a boosted battery voltage greater than the predetermined voltage when a switch provided therein is turned on, and directly outputs the battery voltage when the switch is turned off. A control circuit, which is connected to the constant current circuit, determines whether the battery voltage is greater than the predetermined voltage and turns the switch of the boosting circuit on only when the battery voltage is smaller than the predetermined voltage.